[11.x] Add "previous" state to models

[dshafik]

This change make it possible to retrieve the previously saved state (i.e. what original was before update/save). This gives you the ability to do similar checks as with current vs original state after save.

This adds ->getPrevious($key = null, $default = null) and ->getRawPrevious($key = null, $default = null), as well as syncPrevious() but it might be better to make the latter protected.

I also noticed a delete test in the EloquentUpdateTest so I moved it to the EloquentDeleteTest and added a correct test case in EloquentUpdateTest in its place.

[chu121su12]

Attempted at v9 (#42504). Still a nice to have in core while being an optional trait.

[devfrey]

This has the potential to greatly increase memory utilization for large models and should therefore be opt-in, as suggested in the previous attempt.

[shaedrich]

Would it make sense to have a method to revert the model to the previous state?

[crynobone]

Mark PR as a draft since tests are failing.

[donnysim]

Previous state is really important when working spread out model actions as it becomes extremely hard to track changes. An alternative way, which I prefer and do on my base model, is to disable model automatic "original" syncing and call it manually (which in 99% cases you'll never do or need, though this may have a side effect if you call save many times on the same model) which allows to always access the original data from the moment the model was fetched. Maybe it could also be done as an opt in, which should be easier than tracking previous.